A master 'organiser' has been revealed in human embryonic tissue for the first time by research using human stem cells grafted onto chicken embryos.

The 'organiser', already identified in other mammalian species, is a cluster of cells in the early embryo that act as a master controller of development, instructing the cells around them to develop in certain ways.

Previous attempts to identify this key organisation centre in humans have been limited by current legislation in the UK and internationally, which bans the use of human embryos in research after 14 days, the point at which these cells are expected to appear.

'No one knew what happens after the ball of cells attaches itself to the uterus,' said Dr Ali Brinvalou at Rockefeller University in New York City, who led the study. The research was published in Nature this week.

This work managed to circumvent this ethical restriction through the pioneering development of so-called 'artificial embryos'. The authors discovered that when human embryonic stem cells are grown in small specialised dishes for four days, and are stimulated with the growth signals that would naturally occur in the uterus, cells that express the genetic hallmarks of so-called 'organiser' cells appear.

Scientists then took these organiser cells and grafted them into chicken embryos. They then began to instruct the chicken cells around them to develop into certain cell types, such as the cells of the nervous system. This was alongside the chicken's own organising centre, which was already directing the cells of the embryo in a similar manner, resulting in the development of two nervous systems next to each other.

'To my amazement, the graft not only survived, but actually gave rise to these beautifully organised structures,' Dr Brinvalou said.

Professor Claudio Stern at University College London told Science magazine that the method is 'a nice technical advance', but that he would like to see research conducted beyond the current 14-day time limit in human embryos. 'If we could work on it for a day or two more we could actually study the real organiser.'

Dr Brinavlou agreed the model has limitations. 'There is no substitute for studying the real embryo,' he said. 'Everything else we do when we try to model kind of oversimplifies it.'

Other scientists are hopeful this method will allow new research to fill into human embryonic development; research that had previously been impossible due to the limitations of using human embryos.

Professor Martin Blum, a developmental biologist at the University of Hohenheim in Stuttgart, Germany told Nature News: 'It's a real advance – it's beautiful this can be shown without the need of using embryos.'

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